1. Field of the Invention
The present invention relates to a speed reduction method for a hand-carried engine-driven working machine such as a lawnmower or a chainsaw.
2. Description of the Related Art
Most of engines mounted on hand-carried engine-driven working machines which are currently marketed are expected to use gasoline as a fuel. However, from the viewpoint of reducing the load to the environment, a biofuel which is a mixture of gasoline and bioethanol has started to be propagated, and there are now demands for engines which can deal with such a biofuel.
When a biofuel is used for a gasoline engine, the fuel becomes lean due to the difference in theoretical air-fuel ratio between gasoline and bioethanol. This causes a tendency that the idle speed of the engine is increased or that the speed reduction control on the engine is deteriorated. In particular, in a practical revolution speed range in which a throttle valve of a carburetor is fully opened to increase the revolution speed of the engine to a working revolution speed range, there occurs a so-called lean come-down phenomenon which is a phenomenon in which when the engine is decelerated from a high-speed operating state, the engine has difficulty in reducing its revolution speed down to a clutch-in revolution speed or lower, for example, down to an idle speed range. Once this phenomenon occurs, a working portion such as a rotating blade of the hand-carried engine-driven working machine becomes reluctant to stop, resulting in a dangerous condition.
In addition to the case where the biofuel is used, this lean come-down phenomenon also occurs due to a change in density of air which is triggered by, for example, temperature, atmospheric pressure or elevation, a reduction in internal resistance of the engine associated with aged deterioration, or an increase in amount of air which is triggered by a reduction in airtightness or wear of carburetor parts.
In the lean come-down phenomenon, the phenomenon in which the idle speed of the engine is increased can be dealt with by making use of an approach disclosed in Patent Literature 1 (JP2011-012685), for example. The approach as prior art is such that a revolution speed in an idle revolution speed range is stabilized to a predetermined target revolution speed by controlling an ignition timing (retarding the ignition timing).
In the above-mentioned approach as prior art, however, the acceleration performance of an increasing or decreasing process of the revolution speed of the engine is largely affected, that is, the acceleration in such a change is made difficult to occur. Therefore, the applicable control range is limited to a low revolution speed range including the idle revolution speed range, which is lower than the clutch-in revolution speed which is the lowest revolution speed in the practical revolution speed range of hand-carried engine-driven working machines, leading to a problem that the approach as prior art has difficulty in application to middle to high revolution speed ranges of the practical revolution speed range including the working revolution speed range.